The invention relates to an electrodeless low-pressure mercury vapour discharge lamp with a discharge vessel which encloses a discharge space provided with a filling of mercury and rare gas in a gastight manner, which discharge vessel comprises a light-transmitting enveloping portion and in addition a recessed portion in which a coil for generating a high-frequency magnetic field is enclosed, and which discharge vessel is provided with a luminescent layer at least on a portion of a surface facing towards the discharge space.
Such a lamp is known from EP 0.162.504 B1 (Houkes et al. U.S. Pat. No. 4,710,678). The lamp is operated in that the coil is connected to a high-frequency electric supply source. The magnetic field generated by the coil induces an electric discharge in the discharge space. The coil in addition generates a comparatively strong electric field in the discharge space as a result of potential differences across this coil. The electric field strength may be very great especially near the recessed portion of the discharge vessel. In addition, comparatively high temperatures prevail in the wall of the discharge vessel. The temperature of the recessed portion may even assume values above 200.degree. C.
Under these circumstances, luminescent materials present in the luminescent layer may react with particles from the discharge space which collide with these materials. Depending on the application of the lamp, this may give rise to disadvantages.
Usually, several kinds of luminescent materials are present in the luminescent layer, and the luminous efficacies of these materials are affected by the reactions with the particles to different degrees. The result of this is that the colour point of the light generated by the luminescent layer shows a shift during lamp life. This is no disadvantage when a single lamp is used because this process takes place gradually. Even a comparatively great difference with the colour point at the beginning of lamp life is not observable in that case. Clear differences in colour point, however, are observable in applications where known lamps of mutually differing ages are positioned in one and the same space. If one of the lamps is defective, it is necessary to replace not only the defective lamp, but also the other lamps in said space in order to avoid colour point differences under these circumstances, which is expensive. It was found that differences between colour points are observable when at least one of the colour points is present outside the Von Kries transformed MacAdam ellipse of another colour point (see: New Insights in Chromaticity and Tolerance Areas of Fluorescent Lamps, J. J. Opstelten and G. Rinzema, Journal of the IES, Winter 1987, pp. 117-127). This is the case if there is no ellipse half the size of the transformed MacAdam ellipse which comprises all colour points.
It was found to be favourable to operate the lamp of the kind mentioned in the opening paragraph by means of a pulsatory supply. The lumen output of the lamp may be adjusted between, for example, 10% and 100% of its rated lumen output in this mode of operation in that the ratio of the pulse duration to the time interval between the pulses is varied. In this mode of operation, however, comparatively high voltages are required for re-igniting the lamp at the start of each pulse. Electric fields will then occur, especially near the recessed portion, which are even stronger than those during nominal operation. It was found that mercury is bound to material in the luminescent layer under these circumstances, which mercury is no longer available for lamp operation. A comparatively large quantity of mercury is necessary if a sufficiently long lamp life is to be guaranteed in spite of this. This is bad for the environment in the case of inexpert disposal at the end of lamp life.